1. Field of the Invention
The present invention relates to a semiconductor optical amplifier.
2. Description of the Related Art
In these days, in the advanced scientific region researches using laser light with the pulse time on the attosecond time scale or on the femtosecond time scale, the ultrashort pulse and ultrahigh power laser is actively used. Further, the high power and ultrashort pulse laser diode device that is composed of GaN compound semiconductor and that has light emitting wavelength of 405 nm band has been expected to be a light source for a volumetric optical disc system expected as a next generation optical disc system displacing the blu-ray optical disc system or has been expected to be a light source demanded in the medical field, the bio imaging field and the like.
As the ultrashort pulse and ultrahigh power laser, for example, titanium/sapphire laser is known. Such a titanium/sapphire laser is an expensive and large solid laser light source, which is a main factor to inhibit spread of the technology. If the ultrashort pulse and ultrahigh power laser is realized with the use of a laser diode or a laser diode device, significant miniaturization, price reduction, and high stability are able to be realized, which is expected to become a breakthrough for promoting its wide usage in these fields.
Meanwhile, short pulsation of the laser diode device has been actively researched since 1960s in the communication system field. As a method of generating short pulses in the laser diode device, gain switching method, loss switching method (Q switching method), and mode locking method are known. In these methods, high output is pursued by combining the laser diode device with a semiconductor amplifier, a nonlinear optical device, an optical fiber and the like. The mode locking is further categorized into active mode locking and passive mode locking. To generate light pulses based on the active mode locking, an external oscillator is configured by using a mirror or a lens, and further high frequency (RF) modulation is added to the laser diode device. Meanwhile, in the passive mode locking, light pulses are able to be generated by simple direct current drive by using a laser diode device having a multiple electrode structure.
In the laser light source, obtaining high power is a big challenge. As a means for amplifying light from the laser light source, the semiconductor optical amplifier (SOA) has been keenly examined. The optical amplifier is an amplifier that directly amplifies an optical signal in a state of light without converting the optical signal to an electric signal. The optical amplifier has a laser structure without resonator, and amplifies incident light by light gain of the amplifier.
In the past, the optical amplifier has been mainly developed for optical communication. Thus, for practical application of the semiconductor optical amplifier in 405 nm band, very few precedent cases exist. For example, based on Japanese Unexamined Patent Application Publication No. 5-067845, the semiconductor optical amplifier in 1.5 μm band that uses GaInAsP compound semiconductor and that has a tapered ridge stripe structure has been known. In the technique disclosed in the foregoing Japanese Unexamined Patent Application Publication No. 5-067845, in the semiconductor optical amplifier, a light guide width is gently extended in tapered shape from the narrow input-side-light guide satisfying single mode conditions to output-side-light guide. Thereby, mode field is expanded along the light guide width to improve maximum output of the semiconductor optical amplifier.